1. Field of the Invention
The present invention relates to a method for switching telephone calls from a terminal in a first telephone network to a terminal in a second telephone network.
2. Description of the Prior Art
Private branch exchange systems at different locations within an organization are frequently connected to one another via dedicated lines to form a private network, which forms a first telephone network. This makes it possible for the subscribers to make telephone calls, for example, to one another within the private network without any need to pay charges for an individual call to an operator of a second, preferably public, telephone network.
Private networks allow the costs for exchange calls, that is to say telephone connections from terminals in the private network to terminals in a public telephone network, to be reduced. One service feature for reducing the costs of exchange calls from private networks is also referred to as xe2x80x9cLeast Cost Routingxe2x80x9d, or LCR for short. In this case, many of the connections to terminals in the public telephone network can be transmitted via the private network. In addition, the exit to the public telephone network takes place only in the immediate vicinity of a local network to which a called terminal is connected. A further possibility for reducing costs for exchange calls is to dial a network provider who offers good tariffs for call switching.
The international Laid-Open Specification WO 98/08335 A discloses a separate connection control unit for connection of telephone terminals, in which a connecting route to a dialed subscriber, and a suitable network operator, are selected on the basis of cost-optimized connection data, which is calculated from a likewise disclosed data server based on geographic, statistical and tariff data, and is transmitted to a database in the connection control unit.
The International Laid-Open Specification WO 96/33583 A discloses a least cost router, which is connected to the outputs of a private branch exchange system in a private communications network and, in the event of a connection request within the private communications network, passes this connection request back to a specific input of the private branch exchange system.
The International Laid-Open Specification WO 97/39592 A discloses a communications system having a large number of connected private branch exchange systems, which each contain memory devices for holding connection data, together with implementation units, in order to set up an optimum connecting route via private branch exchange systems in the private communications system and/or switching centers in the public communications systems.
The International Laid-Open Specification WO 94/13109 A discloses a communications system having a large number of through-switching devices, in which case the communications system selects a different connecting path depending on the telephone number dialed by a terminal connected to it. The through-switching devices are each arranged in different tariff zones, so that a connection can be selected on the basis of the level of the charges by a network operator.
The service feature LCR is offered by private networks in which all the private branch exchange systems support this service feature. As such upgrading a private network by adding the service feature LCR, in the prior art, requires either the use of additional hardware devices, extensive changes to programs in each private branch exchange system in the private network, or replacement of all the private branch exchange systems which do not support this service feature by private branch exchange systems which do support this service feature. However, known solutions are highly complex, and are thus associated with major costs.
An object of the present invention therefore is to provide a method for switching telephone calls between different telephone networks using the LCR method and at low cost.
Accordingly, the present invention is based on the idea that the service feature of least cost routing (LCR) can be provided in a first telephone network which contains a number of private branch exchange systems by first of all passing all the connection requests from terminals in the first telephone network to terminals in a second telephone network to a central controller. As soon as the telephone number of a called terminal in the second telephone network is dialed by a calling terminal in the first telephone network, a telephone connection is set up to the central controller. The telephone number is then transmitted to the central controller. The central controller uses the local area code of the dialed telephone number to determine a private branch exchange system from which the transfer to the second telephone network will take place. A telephone connection is then set up to the private branch exchange system which has been determined.
The central controller has a directory, for example in the form of a file, which contains criteria which are taken into account when defining the private branch exchange system from which the first telephone network will be left. As already explained, the local area code is used to determine a private branch exchange system in the immediate vicinity of the destination. A further criterion may be the time of the connection request if particularly good tariffs are offered by different network providers at specific times of day or on specific days of the week.
New or amended tariffs from network providers can be taken into account particularly easily with the method according to the present invention, since only the directory in the central controller is adapted in the event of tariff changes. In telephone networks in which the service feature LCR is supported by all the private branch exchange systems, tariff changes, on the other hand, require adaptations in all the private branch exchange systems.
The present invention allows the service feature LCR to be offered in telephone networks in which the service feature LCR is supported only by the central controller. Private branch exchange systems in telephone networks which do not support this service feature are adapted such that they pass connection requests to subscribers in a second telephone network to the central controller rather than directly to the second telephone network. This adaptation can be carried out with little complexity in the private branch exchange systems. Particularly in telephone networks having relatively old private branch exchange systems, the method according to the present invention avoids the relatively old private branch exchange systems having to be replaced by new private branch exchange systems.
The first telephone network may be, for example, a private network of an organization. As mentioned initially, the private branch exchange systems are typically located at different sites in the organization, and are connected to one another via tielines. The second telephone network may be, for example, a public telephone network, which is operated by a network operator.
In one preferred embodiment, each private branch exchange system is configured such that it sets up a connection to the central controller immediately after an exchange code character has been dialed. The other characters in the telephone number are transmitted to the central controller once they have been dialed by the subscriber. The central controller starts to evaluate the character sequence of the telephone number as soon as the last character has been dialed and transmitted. The time which a subscriber needs to dial the entire character sequence is considerably longer than the time which is required to set up the connection to the central controller and to evaluate the character sequence. The setting up of the connection to the called terminal is thus delayed only slightly.
The method is also suitable for private networks in which the private branch exchange systems have associated code characters. These code numbers are used, for example in conjunction with telephone number directories, for passing on telephone connections within the private network. Each private branch exchange system has a telephone number directory which allocates a cross-connection to a code number, and this cross-connection connects the private branch exchange system to an adjacent private branch exchange system.
In one advantageous development of the method according to the present invention, the code number of the private branch exchange system which has been determined and the character sequence of the dialed telephone number are used to form a new character sequence. The central controller contains a program module for forming the new character sequence. The telephone connection to the second telephone network is set up using the new character sequence. The advantage of this development is that the telephone number directories which already exist in the private branch exchange systems are used for switching the telephone calls. This is advantageous in particular when the respective shortest routes determined from the telephone number directories in the private branch exchange systems are defined between the private branch exchange systems in the private network.
In one preferred embodiment, the central controller is integrated in a private branch exchange system. This embodiment offers the advantage that a telephone connection is set up between the central controller and the private branch extension which is being determined and is used to continue the connection from the calling private branch extension to the central controller. This embodiment is feasible, for example, when the private branch exchange systems in the first telephone network are connected to the central controller in the form of a star.
However, the central controller also may be configured such that it transmits the new character sequence to the calling private branch exchange system. The calling private branch exchange system uses the new character sequence to set up a connection to the private branch exchange system which has been determined, and from which the transfer to the public telephone network takes place. This refinement of the method according to the present invention relieves the load on the central control of having to set up connections to the private branch exchange system which has been determined.
It is particularly advantageous if, in the already described refinement of the method according to the present invention, the connection between the calling private branch exchange system and the central controller is cleared once the new character sequence has been transmitted. This ensures that no superfluous lines between the private branch exchange systems are occupied in the first telephone network.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Preferred Embodiments and the Drawings.